1. Field of the Invention
This invention relates to an inner rotor-type brushless motor having a slide bearing.
2. Description of the Related Art
Many electronic devices acquire power from a motor. In recent years, with the reduction in size and price and the increase in accuracy of the electronic devices, demand has increased for smaller, inexpensive and more accurate motors used in the electronic devices.
A sectional view, as taken in axial direction, of the structure of the conventional inner rotor-type brushless motor is shown in FIG. 7.
The brushless motor shown in FIG. 7 includes a rotor A with a rotary shaft member A1 inserted thereon, bearings B1, B2 for rotatably supporting the rotor A, a stator C, a circuit board D and covers E1, E2 for accommodating these component parts.
The rotor A has a structure in which the rotary shaft member A1 fitted in an inner through hole of a cylindrical rotor core A2, and a magnet F is fitted on the outer peripheral surface of the rotor core A2.
The stator C is arranged in radially slightly spaced relation with the magnet F. The stator C is formed by turning a winding C2 around the stator core C1 configured of a stack of a plurality of lamination layers.
The circuit board D is arranged under the stator C as viewed in axial direction. The circuit board D includes a control IC D1 and a position sensor D2 for detecting the magnetism. The position sensor D2 is arranged in radial direction in spaced and opposed relation with the magnet F. The position sensor D2 detects the magnetism of the magnet F and sends a position detection signal to the control IC D1. Based on the position detection signal, the control IC D1 controls the current supplied to the winding C2 and thus controls the motor rotation.
At positions above and under the rotor core A2 as viewed in axial direction, bearings B1, B2, respectively, are inserted on the rotary shaft member A1. The bearings B1, B2 rotatably support the rotor A.
To accommodate these component parts, the covers E1, E2 are arranged. The cover E1 has the shape of a substantially bottomed cylinder, and is fixed with the cover E2 having the shape of a substantially bottomed cylinder at the cylindrical end surface of the cover E1. The covers E1, E2 both have a central recess. The recesses accommodate the bearings B1, B2, respectively, internally fitted therein. The stator C is also accommodated fixedly with an adhesive or the like in the cover E1.
In the structure of the inner rotor-type brushless motor shown in FIG. 7, however, the rotor core A2 is not hollow, and therefore, the bearings B1, B2 are required to be arranged at positions above and under the rotor core A2 as viewed in axial direction. To accommodate the bearings B1, B2, therefore, the covers E1, E2 are required to be correspondingly larger. This also makes it difficult to reduce the size of the motor.
Further, the two covers E1, E2 of the motor are required to be fabricated to cover the motor. This increases both the number of parts and the cost of motor fabrication, thereby preventing the most cost from being reduced. In the case where the bearings B1, B2 are held by the covers E1, E2 which are different parts, the assembly error of the covers E1, E2 constituting different parts combines with the dimensional error of each unit part thereby to deteriorate the coaxiality of the rotary shaft member A1. Specifically, the center of the bearings B1, B2 are respectively deviates from the rotational shaft, and the rotary shaft member A1 is undesirably supported at an angle. As a result, the rotary shaft member A1 deviates from the rotational center axis and wobbles, thereby often having an adverse effect on a mating part to which the force is transmitted from the rotary shaft member A1.
In the case where a gear is fitted on the forward end of the rotary shaft member A1 to transmit the force through a mating gear in mesh, for example, a low coaxiality of the rotary shaft member A1 would lead to an unsatisfactory engagement with the mating gear, and imposes an offset load on the gears of both the motor and the mating part. Once at least one of the gears of the motor and the mating part is damaged, the gear may be cut or dented. Further, the cut or the dent may cause a bite failure, generating vibrations and noises. In the worst case, the gear may be broken, thereby causing a fatal malfunction of the electronic device carrying the motor.